1. Field of the Invention
This invention relates in general to improvements in the field of computer systems having backup/restore or archive/retrieve subsystems, and more particularly, to a method for implementing point-in-time copy operations using snapshot functions.
2. Description of Related Art
In a data processing system, a backup/restore subsystem is typically used to save a recent copy or version of one or more data sets or portion thereof on some form of backup data storage device, such as magnetic or optical disk drives, tape drives, or other memory. The backup/restore subsystem is used to protect against loss of data. For example, if an on-line version of one or more data sets is destroyed, corrupted, deleted, or changed because of power failure, hardware, or software error, user error or some other type of problem, the latest version of those data sets which are stored in a backup/restore subsystem can be restored and therefore the risk of loss of data is minimized. It is readily apparent that backup/restore subsystems are and will remain an important part of the field of data processing.
Successful recovery of data to a known consistent state requires a backup of all components of the data at the same consistent point in time. Generally, a point in time backup is a copy of the data which is logically consistent to a given point in time, with the: restriction that the amount of time to obtain logical consistency is significantly less than the amount of time to actually copy the data.
Concurrent copy, also known as time-zero copy, provides the ability to create a point-in-time backup. Concurrent copy is a point in time backup which uses a combination software and microcode architecture to obtain a copy of the original data at the time the backup was initiated. However, concurrent copy is usually only supported for data residing on storage subsystems which implement the concurrent copy functions.
One storage subsystem that does not support concurrent copy functions is the log-structured array subsystem (LSA). An LSA subsystem implements "virtual volumes", wherein each virtual volume is created using a "virtual track table" having pointers to "virtual tracks" (i.e., records) in a sequential byte stream, wherein updated tracks are written to a new location at the logical end of the byte stream and their associated pointers, are reset to the new locations. Thereafter, the tracks at the old location in the sequential byte stream are no longer needed and can be released as free space for reclamation and reuse.
In certain products, emulation functions performed by the LSA subsystem permit computer programs executed by a host processor coupled to the LSA subsystem to "view" the LSA subsystem as a standard direct access storage device (DASD) with sequentially numbered tracks. However, such emulation does not necessarily extend to all functions generally capable of being supported with standard DASD.
Currently available LSA subsystems do support a very fast copy function called a snapshot function, which operates by copying pointers between virtual track tables representing different virtual data volumes without actually moving any data. On the other hand, the snapshot function suffers from a restriction that the source and target locations must be within the subsystem, and the same device types. As a result, a straightforward backup copy using current snapshot functions does not achieve point-in-time backup on LSA subsystems.
Thus, there is a need in the art for methods for providing point in time backups on LSA subsystems, wherein the point-in-time backups use snapshot functions.